Gold electroplating baths and process

ABSTRACT

A gold electroplating bath for use in barrel plating, rack plating operations or the like. The bath comprises an aqueous solution of a gold-alkali metal cyanide, conducting and buffering compounds, quantities of aminoguanidine, and one or more agents selected from the bath soluble salts and complexes of lead, thallium, and/or arsenic.

United States Patent Rick et al.

[451 Sept. 3, 1974 GOLD ELECTROPLATING BATHS AND PROCESS Inventors: Maurice Bick, South Orange; Jean A. Lochet, Metuchen, both of NJ.

Related US. Application Data Continuation-impart of Ser. No. 176,263, Aug. 30, 1971, and a continuation-in-part of Ser. No. 273,860, July 21, 1972.

US. Cl. 204/46 G, 204/43 G Int. Cl. C23b 5/28, C23b 5/46 Field of Search 204/46, 43 G; 106/1 References Cited UNITED STATES PATENTS 2/1955 Spreter et al. 204/44 4/1968 Danemark et al. 204/44 1/1969 Greenspan 204/43 G 3,475,290 10/1969 Yamamura et a1. 204/43 G 3,520,785 7/1970 Duva et al. 204/43 G 3,562,120 2/1971 Duva et al. 204/46 3,637,473 l/1972 Greenspan et a1 204/46 3,644,184 2/1972 Smith et al 204/46 3,666,640 5/1972 Smith 204/44 3,672,969 6/1972 Nobel et a1. 204/43 G 3,753,874 8/1973 Zimmerman et al. 204/43 G FOREIGN PATENTS OR APPLICATIONS 1,102,179 2/1968 Great Britain 204/46 Primary Examiner-G. L. Kaplan Attorney, Agent, or Firm-Stefan J. Klauber nide, conducting and buffering compounds, quantities of aminoguanidine, and one or more agents selected from the bath soluble salts and complexes of lead, thallium, and/or arsenic.

21 Claims, No Drawings GOLD ELECTROPLATING RATES AND PROCESS This application is a continuation-in-part of our copending applications, Ser. No. 176,263, filed Aug. 30, 1971, now abandoned, for GOLD AND GOLD ALLOY ELECTROPLATING BATH, and Ser. No. 273,860, filed July 21, 1972, now US. Pat. No. 3,770,596, for GOLD PLATING BATH FOR BAR- REL PLATING OPERATIONS, which applications are assigned to the same assignee as the present application.

BACKGROUND OF INVENTION This invention relates generally to electroplating baths and compositions, and more specifically relates to such baths and compositions as are useful in the electroplating of gold.

Numerous gold electroplating compositions are known in the art, which function in the presence of mildly acidic conditions and yield deposits of relatively good quality. Many of the heretofore reported acidic baths, however, have tended to provide deposits of gold which were lacking in uniformity of color. This result, at least in part, is believed to obtain because of nodular buildup during the course of deposition. The color and brightness of many of the deposits yielded by the said prior baths have, furthermore, been aesthetically displeasing.

Yet another undesirable aspect of many prior art acidic gold electroplating baths, has been that the efficiency achieved during the electroplating process is relatively low. In an effort to improve upon this last cited problem, it has recently been proposed (in US. Pat. No. 3,669,852), to introduce a reducing agent, such as hydrazine or a salt of hydrazine into the plating bath. This agent is said to remove dissolved oxygen during the plating process, in consequence of which an improvement in the efficiency of the process is said to occur. The said efficiency, however, does not truly approach the desired 100 percent which is ideally sought after.

It may also be noted in connection with some of the foregoing problems, that there has recently been proposed (in U.S. Pat. Nos. 3,562,120 and 3,644,184) that a soluble thallium compound be incorporated in an acidic gold electroplating bath, for improving the distribution in the resultant plating. Baths of this type, however, while indeed improving the resultant plating distribution in applications such as barrel plating, still possess many of those previously indicated deficiencies, e. g., the said baths are found to be relatively inefficient. Thus in baths of the type taught by the said patents, the efficiency typically may be of the order of only about 90 percent.

In our aforementioned copending application Ser. No. 176,263 we disclose our finding that the additive aminoguanidine appears to function in an acid gold plating solution to improve the mechanical and aesthetic properties of a resultant plating. A possibly related finding is also reported in British Pat. No. 1,102,169 where the said aminoguanidine performs what may be a related function in the brightening of gold deposits.

Further, in our aforementioned copendingapplication Ser. No. 273,860, we disclose a bath based on a chelating phosphonic acid, with quantities of hydrazine and small amounts of arsenic and lead ion. This bath minimizes bipolarity effects which can occur during, e.g., barrel plating, and otherwise results in improved distribution of deposited metal. The said bath, however, suffers from a tendency to decompose, especially at an acid pH of around 4, a result which limits the utility of the bath.

In accordance with the foregoing, it may be regarded as an object of the present invention, to provide a gold electroplating bath, which operates at efficiencies approaching 100 percent, and provides platings of improved lustre, metal distribution, and possessing aesthetically pleasing appearances.

It is a further object of the present invention, to provide a gold electroplating bath for use in barrel plating operations or the like, which functions with a high degree of efficiency over an extended pH range, including regions of relatively high acidity, up to relatively elevated temperatures, and which moreover displays outstanding stability characteristics.

It is a further object of the invention, to provide a gold electroplating bath which produces deposits having good surface quality, with relatively slight nodule buildup, and which enables production of uniform and stress-free deposits, even where the deposits are of relatively high thickness.

SUMMARY OF INVENTION Now in accordance with the present invention it has been discovered that theforegoing objects, and others as will become apparent in the course of the ensuing specification, are achieved in a plating bath incorporating quantities of aminoguanidine, in a gold plating solution containing gold in a form of an alkali metal cyanide, a brightener and efficiency-augmentingagent in the form of one or more bath soluble salts or chelates of lead, thallium, and/or arsenic, together with conducting and buffering compounds in quantities appropriate to provide a specific gravity for the bath of about 6 to 20 Baume. The bath is effective in use at temperatures as low as 50C, and more generally in the range of 50 to C. Apreferred specific gravity for the bath is from about 12 to 16 Baume. In preparing the baths of the invention one adds the conducting and buffering compounds, which include one or more partially neutralized acids, in quantities which yield a bath specific gravity appropriate to the bath application and for the said acids which are present. Thereafter, the pH of the bath, as required, may be adjusted by addition of small quantities of innocuous acids or bases as is known in the art.

Where baths of the foregoing type are utilized in electroplating of gold, it is found that the efficiencies achieved approach a true percent. Although it is not completely understood why such high efficiencies are enabled, it is speculated that a synergistic effect of some type occurs among the above-cited components. This is particularly believed to be so, in that when a bath of the character specified includes the aminoguanidine but not the brightener and efficiencypromotor, an efficiency well below 100 percent is observed; whereas upon addition of the specified small amount of arsenic, lead or thallium components, the bath efficiency rises well beyond that which would normally be expected, and indeed approaches the indicated figureof 100 percent even at temperatures as low as 50C.

The deposits of gold achieved'inaccordance with the invention are found to be substantially pure gold, and are found to be accepted readily to formation of a goldsilicon eutectic. The combination of the invention appears to provide an inhibiting effect, which'prevents codeposition of the arsenic, lead, and/or thallium additives. The said deposits are eminently suited for application to the semi-conductor and electronic industries. Furthermore, the deposits are markedly smooth, free of nodules, and display excellent distribution, all of which are factors of considerable interest, for example, in the electro-forming of or the plating of very thick plates. Distribution, in particular, is a most significant factor in reducing the costs of gold plating, a factor of everincreasing interest in arising price gold market.

DESCRIPTION OF PREFERRED EMBODIMENTS may be consumed during electrolysis, and must be replenished in that if the concentration level becomes too low, erratic results will be obtained. The aminoguanidine bicarbonate can be added as such to acidic baths, but is preferably reacted with sulfuric acid, citric acid, or any suitable acid compatible with the plating bath, when the bath in which the aminoguanidine is to be incorporated has a pH equal to or greater than 7.0.

Arsenic may be added to the bath in the form of potassium or sodium arsenite; similarly thallium may be added as. the salt thallium sulfate, or in the form of a bath-soluble complex. Other bath soluble complexes or salts are also suitable. The addition of a soluble form of lead is somewhat more difficult, and it is preferably provided in the form of a suitable complex, such as one formed with EDTA or with ethylenediamine tetra methylene phosphonic acid (EDMPA). Where arsenic is utilized its effects on brightness alone are evident throughout the pH range of about 6.0 to 8.5. The efficiency-promoting aspects of the arsenic, however, are not fully apparent outside a pH range of from about 5.0 to 7.5 This latter range is therefore a preferred one for fully realizing the effects of the invention where arsenic is used. Thallium, however, yields the desired brightening and efficiency augmenting effects over a much wider pH range, extending from about 3.0 to 12.0, with lead having a similar range of application to that noted for arsenic.

The remarkable effects of aminoguanidine addition upon the efficiency characteristics of the baths utilized in the invention, are illustrated by first considering the following Example I. In particular, a bath was prepared with the following components:

The bath of Example I was used over an extended period in production, and was found to have an efficiency of only, 106 mg/amp-min. (85 percent), The bath during this period was used normally, carbon treated regularly and so forth. The gold content was about 10 g/l. Plating voltage was about 2 volts for current density of 3 A.S.F. After an addition of 10 g/l of aminoguanidine bicarbonate, the voltage dropped to about 1.3 volts, and the efficiency rose to I22 mg/amp-min. (about 99 percent).

This Example I bath has been used to provide stressfree deposits of several mils thickness, the resulting deposit being uniform, free of nodules, etc. for the entire thickness.

Example II Dipotassium phosphate 100 g/l Monopotassium phosphate 40 g/l Boric acid 60 g/l Aminoguanidine bicarbonate 25 g/l As*** 2 mg/l Gold 8-10 g/l pH 7.0

In this, as in other Examples, the pH of the bath may be adjusted to its indicated values by addition of KOH or phosphoric acid, or preferably by addition of the major component acid used in the formulation e.g'., by adding citric acid to the bath of Example land so forth. The bath in Example II has been found to be eminently suitable for barrel plating of complex parts such as TO-S headersfl'he metal distribution in the resultant plating is excellent. Tl or Pb can be substituted for the As***, inaccordance with the invention. The significance of these agents and the apparent synergistic action that occurs, is illustated by the fact that at a temperature of C, satisfactory results are not achievable without the said metallic agents. Furthermore, it is notedthat when such metals are added to the bath in minute quantities, the efficiency rises abruptly l to 2 percent or more. Again itis hypothesized here that some type of synergistic action occurs between the aminoguanidine and the metallic components, whereby the efficiency of the resultant combination approaches 100 percent. y

' In general, the level of addition of the brightener and efficiency-augmenting agent utilized in the present baths, will be in concentrations sufficient to provide an increase in the efficiency of the bath and improved surface qualities for the resultant plating in comparison to the corresponding characteristics obtained in the absence thereof. When the said brightener and efficiencyaugmenting agent utilized with the baths of the invention comprises arsenic, a preferred range of addition is from 2 to 12 mg/l of solution, with from 2 to 4 mg/l being optimum, and useful results being achieved with as little as 0.6to 0.8 mg/l. Both lead and thallium may be utilized from about 1 to 30 mg/l concentrations, or somewhat higher. A preferred range of addition for thallium is from 4 to 8 mg/l; and for lead, around 12 to 20 mg/l (e.g., where lead is in the form of an EDTA complex). The gold itself, in the form of an alkali metal gold cyanide, is present in the baths of the invention in concentrations providing from about 2 to 40 g/l of gold calculated as the metal.

Similarly in connection with Example II, it is found that when the aminoguanidine is added to the bath, a

significant voltage drop occurs. For example, utilizing the bath of Example I to plate a brass panel 1% inches X 1 inch with a current of 50 ma, the voltage is 1.8 volts (temperature 65, moderate agitation). When 5 grams of aminoguanidine are added to the bath the voltage drops to about 1.1 volt. A similar effect is achieved with other baths. The results achieved by the addition of aminoguanidine bicarbonate is permanent, providing the concentration of the additive is maintained and, of course, that the other variables of the bath are maintained during the normal range. Buildup of the decomposition products is rectified by removal, utilizing periodic and regular carbon treatment, as is known in the art.

EXAMPLE Ill This Example illustrates a bath in accordance with the invention which utilizes arsenic, and wherein a relatively high pH is achieved:

Dipotassium phosphate 150 g/l Aminoguanidine bicarbonate g/l Gold 8 g/l pH, adjusted with KOH or phosphoric acid, to 8.3 Plating temperature 50C In preparing this bath the aminoguanidine bicarbonate was reacted before addition to the bath, with phosphoric acid. A panel plated with the bath at 50C and at 50 ma (current density about 3 A.S.F.) displayed a good, semi-bright deposit. Thereafter 4 mg/l of As were added to the bath. A plating conducted under similar conditions to the above yielded a deposit that was almost fully bright, and having generally superior surface qualities. The efficiency of this modified bath (i.e., with the addition of the As'**) was 123 mg/AM, which is approximately 100 percent.

It should be pointed out that the utilization of aminoguanidine bicarbonate is not restricted to use with phosphate or citrate buffer systems. A wide variety of acids and their soluble salts can be utilized, such as the borates, sulfamates, pyrophosphates, organophosphorous compounds such as hydroxymethylphosphonic acid (available from Stauffer Chemicals), ethylenediamine tetra methyl phosphonic acid (EDMPA) available from Monsanto (under the designation Dequest 2041), EDTA and related compounds, glycine, diacetic acid and derivatives such as N- phosphonomethyl-aminodiacetic acid, the bath soluble salts of the cited acids, and the like. It has also been found that the addition of a chelating agent to the formulations, yields desirable results. It is hypothesized here that a synergistic action occurs between the chelating agent and aminoguanidine, which tends to prevent or slow down the codeposition of troublesome impurities, such as lead, copper,'nickel, iron and cobalt. The preferred species of chelating agents are the organo-phosphorus chelating agents such as the cited EDMPA, etc., the aminopolycarboxylic chelating agents such as EDTA, CDTA, NTA, etc. Glycine and DHG can also be of value. Among the organic acids that may be used, are such weak organic acids as acetic acid, formic acid, malic acid, succinic acid, and the like, in addition to the citric acid already cited.

The bath of Example II is well suited to the plating of multi-lead headers, such as the well-known TO-S headers, and results in well plated objectslacking in bare spots. The said bath thus appears to be highly effective in eliminating the undesirable effects of polarization, which are well-known to occur in barrel plating operations. It may be noted in this connection that in our copending cited application Ser. No. 273,860, we disclose baths'utilizing in addition to other components, hydrazine additives which appear to be very useful in preventing the cited bipolarity effect. The baths of the present invention are superior to those cited baths, particularly at the relatively acidic pH range as, for example, in the range of 4 to 4.5 pH which is a customary region of acidity for operation of the strike baths utilized to pre-coat Kovar and the like. This aspect of the present invention is illustrated by the following example.

Example IV Dipotassium phosphate I00 g/l EDTA 50 g/l Potassium hydroxide to adjust pH Aminoguanidine bicarbonate 25 g/l Boric acid g/l Gold 4 g/l pH 4.2 Thallium (as metal) 4 mg/l Plating temperature C A further example of a strike-type bath is set forth in Example V below: 7

Example V Gold 4 g/l Sulfamic Acid g/l Sodium salt of dithdroxethylglycine (DHG) 100 g/l Aminoguanadine bicarbonate 25 g/l Thallium (as metal) 10 mg/l pH Adjusted to 4.0 Plating temperature 65C Example VI EDMPA 100 g/l KOH 45 g/l Aminoguanidine bicarbonate 10 g/l Gold 5 g/l pH adjusted to 4.5

A panel plated in the bath of Example Vl at a current density of 4 ASP and at a temperature of 50C was found to be brown in color and not deemed acceptable. Efficiency was about mg/amp-min. 2 mg of thallium per liter in the form of a solution of thallium sulfate was then added to the bath, and a further panel plated under identical conditions as above. The resultant gold coating was found to be of a pleasing smooth, yellow color of a high quality. The efficiency was further found to be about 123 mg/amp-min. (approaching 100 percent).

While the present invention has been particularly set forth in terms of specific embodiments thereof, it will be understood in view of the instant disclosure that numerous variations upon the invention are now enabled to those skilled in the art, which variations yet reside within the scope of the present teaching. Accordingly, the invention, is to be broadly construed and limited only by the scope and spirit of the claims now appended hereto. 3 I

We claim: 1. An electroplating bath for ing gold, comprising: i

an aqueous solution of a gold-alkali metal cyanide, said gold being present fromabout 2 g/l to about 40 g/] calculated as the metal; from about 1 to 50 g/l aminoguanidine, calculated as the bicarbonate; one or more agents selectedfrom the group consisting of the bath soluble salts and complexes of lead, thallium and arsenic, said agent being present in concentrations sufficient to provide an increase in the efficiency of said bath and improved surface qualities for resultant platings in comparison to the correspondingcharacteristics obtained in the absence thereof; and additional conducting and buffering compounds including one or more partially neutralized acids, in concentrations appropriate in the presence of the remaining bath components to adjust the bath specific gravity to a desired'level of about 6 up to Baume, said level being above that which would be achieved in the absence of said compounds. 2. A bath in accordance with claim 1, wherein said specific gravity is in the range of from about 12 to 16 Baume.

electrolytically deposit- 3. A bath in accordance with claim 2, wherein said acid comprises a weak organic acid.

4. A bath in accordance with claim 3, wherein said acid comprises citric acid.

5. A bath in accordance with claim 2, wherein said acid comprises a chelating phosphonic acid.

6. A bath in accordance with claim 5, wherein said phosphonic acid comprises ethylenediamine tetra methylphosphonic acid.

7. A bath in accordance with claim 2, wherein said acid comprises a chelating aminocarboxylic acid.

8. A bath in accordance with claim 7, wherein said acid comprises EDTA.

9. A bath in accordance with claim 2, wherein said acid comprises hydroxymethylphosphonic acid.

10. A bath in accordance with claim 2, wherein said acid comprises dihydroxyethylglycine.

, A 8 I 11. A bath in accordance with claim 2, wherein said acid comprises N-phosphonomethyliminodiacetic acid.

12. A bath in accordance with claim 2, wherein said acid comprises boric acid.

13. A bath in accordance with claim 2, wherein said aminoguanidine is present in the range of from 4 to 30 g/l. t

14. A bath in accordancewith claim 2, wherein said acid comprises sulfamic acid.

15. A bath in accordance with claim 1, wherein said aminoguanidine is present in the range of from about 4 to 30 g/l.

16. A bath in accordance with claim 1, wherein said agent comprises thallium.

17. A bath in accordance with claim 1, wherein said agent comprises lead.

18. A bath in accordance with claim 1, wherein said agent comprises arsenic.

19. A method for electroplating of complex parts with gold, comprising:

barrel plating said complex part in an aqueous solution of a gold alkali metal cyanide, said gold being present from about 2 g/l to about 40 g/l calculated as the metal;

said bath further including from about 1 to 50 g/l of aminoguanidine calculated as the bicarbonate; one or more agents selected from the group consisting of the bath soluble salts and complexes of lead, thallium, and arsenic, said agent being present in concentrations sufficient to provide an increase in the efficiency of said bath during said plating, and improved surface qualities for resultant platings in comparison to the corresponding characteristics obtained in the absence of said agents; and additional conducting and buffering components including one or more partially neutralized acids, in concentrations appropriate in the presence of the remaining bath components to adjust the bath specific gravity to a desired level of about 6 up to 20 Baume, said level being above that which would be of from about 4 to 30 g/l. 

2. A bath in accordance with claim 1, wherein said specific gravity is in the range of from about 12* to 16* Baume.
 3. A bath in accordance with claim 2, wherein said acid comprises a weak organic acid.
 4. A bath in accordance with claim 3, wherein said acid comprises citric acid.
 5. A bath in accordance with claim 2, wherein said acid comprises a chelating phosphonic acid.
 6. A bath in accordance with claim 5, wherein said phosphonic acid comprises ethylenediamine tetra methylphosphonic acid.
 7. A bath in accordance with claim 2, wherein said acid comprises a chelating aminocarboxylic acid.
 8. A bath in accordance with claim 7, wherein said acid comprises EDTA.
 9. A bath in accordance with claim 2, wherein said acid comprises hydroxymethylphosphonic acid.
 10. A bath in accordance with claim 2, wherein said acid comprises dihydroxyethylglycine.
 11. A bath in accordance with claim 2, wherein said acid comprises N-phosphonomethyliminodiacetic acid.
 12. A bath in accordance with claim 2, wherein said acid comprises boric acid.
 13. A bath in accordance with claim 2, wherein said aminoguanidine is present in the range of from 4 to 30 g/l.
 14. A bath in accordance with claim 2, wherein said acid comprises sulfamic acid.
 15. A bath in accordance with claim 1, wherein said aminoguanidine is present in the range of from abouT 4 to 30 g/l.
 16. A bath in accordance with claim 1, wherein said agent comprises thallium.
 17. A bath in accordance with claim 1, wherein said agent comprises lead.
 18. A bath in accordance with claim 1, wherein said agent comprises arsenic.
 19. A method for electroplating of complex parts with gold, comprising: barrel plating said complex part in an aqueous solution of a gold alkali metal cyanide, said gold being present from about 2 g/l to about 40 g/l calculated as the metal; said bath further including from about 1 to 50 g/l of aminoguanidine calculated as the bicarbonate; one or more agents selected from the group consisting of the bath soluble salts and complexes of lead, thallium, and arsenic, said agent being present in concentrations sufficient to provide an increase in the efficiency of said bath during said plating, and improved surface qualities for resultant platings in comparison to the corresponding characteristics obtained in the absence of said agents; and additional conducting and buffering components including one or more partially neutralized acids, in concentrations appropriate in the presence of the remaining bath components to adjust the bath specific gravity to a desired level of about 6* up to 20* Baume, said level being above that which would be achieved in the absence of said components.
 20. A method in accordance with claim 19, wherein the specific gravity of said bath is in the range of 12* to 16* Baume.
 21. A method in accordance with claim 20, wherein said aminoguanidine is present in said bath in the range of from about 4 to 30 g/l. 